Await a synchronous iterator's final value in for await

for await over a synchronous iterator held as any reads and awaits the value
of the result that finishes the loop, as the async-from-sync iterator does, so
a throwing value getter or a rejected promise there rejects the loop.

packages/compiler/src/frontend/lowering/lower-stmts.ts

@@ -10807,8 +10807,8 @@ function lowerForOfDyn(
 }
 
 /** One step of `for await` over a checked-dynamic iterator: await next()'s
- * result, read done before value, and await the value of a synchronous
- * iterator, as the async-from-sync iterator does. */
+ * result, read done before value, and await each value of a synchronous
+ * iterator, including its final one, as the async-from-sync iterator does. */
 function asyncIteratorValue(
   lowerer: Lowerer,
   iterator: IrExpr,
@@ -10866,14 +10866,29 @@ function asyncIteratorValue(
         loc,
       },
     ],
+    // A synchronous iterator's final value is still read and awaited, so a
+    // throwing getter or a rejected promise there rejects the loop.
     result: {
       kind: "ternary",
-      cond: varRef(done.id, BOOL, loc),
-      then: dynUndefinedExpr(loc),
+      cond: sync,
+      then: {
+        kind: "ternary",
+        cond: varRef(done.id, BOOL, loc),
+        then: {
+          kind: "seqExpr",
+          stmts: [{ kind: "exprStmt", expr: awaitDyn(get("value")), loc }],
+          result: dynUndefinedExpr(loc),
+          type: DYN,
+          loc,
+        },
+        else_: awaitDyn(get("value")),
+        type: DYN,
+        loc,
+      },
       else_: {
         kind: "ternary",
-        cond: sync,
-        then: awaitDyn(get("value")),
+        cond: varRef(done.id, BOOL, loc),
+        then: dynUndefinedExpr(loc),
         else_: get("value"),
         type: DYN,
         loc,

tests/corpus/6715-dyn-for-await-protocol.ts

@@ -1,6 +1,25 @@
 // for await and async yield* over values held as any: a Symbol.asyncIterator
 // method that returns a compiled generator, a synchronous delegate whose
-// final value is a promise, and return() results that are not objects.
+// final value is a promise, a synchronous iterator whose final value throws
+// or rejects, and return() results that are not objects.
+const settle: any = (value: unknown, fails: boolean) =>
+  new Promise((resolve, reject) => (fails ? reject(new Error(String(value))) : resolve(value)));
+function finishing(last: () => any): any {
+  return {
+    [Symbol.iterator]() {
+      let step = 0;
+      return { next: () => (step++ < 1 ? { value: 1, done: false } : last()) };
+    },
+  };
+}
+async function loop(label: string, source: any): Promise<void> {
+  try {
+    for await (const value of source) console.log(label, "value", value);
+    console.log(label, "completed");
+  } catch (error) {
+    console.log(label, "caught", (error as Error).message);
+  }
+}
 async function main(): Promise<void> {
   const generated: any = {
     async *[Symbol.asyncIterator]() {
@@ -56,5 +75,19 @@ async function main(): Promise<void> {
   } catch (error) {
     console.log("return", (error as Error).name, (error as Error).message);
   }
+
+  await loop("getter", finishing(() => ({ done: true, get value(): number { throw new Error("value getter"); } })));
+  await loop("rejected", finishing(() => ({ value: settle("rejected final", true), done: true })));
+  await loop("resolved", finishing(() => ({ value: settle(9, false), done: true })));
+  await loop("rejected step", finishing(() => ({ value: settle("rejected step", true), done: false })));
+  async function* finalRejects(): AsyncGenerator<unknown, void> {
+    const result = yield* finishing(() => ({ value: settle("delegate final", true), done: true }));
+    console.log("delegate completed", result);
+  }
+  try {
+    for await (const value of finalRejects()) console.log("delegate value", value);
+  } catch (error) {
+    console.log("delegate caught", (error as Error).message);
+  }
 }
 main();

tests/harness/corpus-costs/6715-dyn-for-await-protocol.ts.json

@@ -1 +1 @@
-{ "plain": 3200, "san": 3800 }
+{ "plain": 3200, "san": 4100 }